Can the Intermediate Western Boundary Current recirculation trigger the Vitória Eddy formation?

Abstract

South of the Vitória-Trindade Ridge, a seamount chain off East Brazil, the Brazil Current (BC) meanders cyclonically within Tubarão Bight, occasionally forming the Vitória Eddy. It was recently found that the Intermediate Western Boundary Current (IWBC), which flows equatorward below the BC, cyclonically recirculate within Tubarão Bight. We present an analysis of AVISO observations that suggest that the Vitória Eddy formation is conditioned by the strength of the BC upstream of Tubarão Bight. A weak BC is prone to local meandering and eddy formation in the bight, while a strong BC suppresses eddy formation in the bight but triggers downstream meander growth. To study the effects of the IWBC recirculation on the BC meandering and the Vitória Eddy formation, we formulate a simple two-layer quasi-geostrophic model. In the model, the BC is represented by a meridional jet in the upper layer and the IWBC recirculation is a steady eddy in the lower layer. The lower-layer eddy effectively acts as a topographic bump, affecting the upper-layer jet via the stretching term ψ 2 /R 2 d , where ψ 2 is the lower-layer streamfunction and R d is the baroclinic deformation radius. Based on the AVISO sea-surface height data and previous observational studies, we define a stationary eddy and reference jet. We conduct a number of initial-value problem experiments varying the upper-layer jet speed. A weak upper-layer jet slowly meanders and develops a cyclone above the lower-layer eddy. As we increase the jet velocity, the meandering is faster and the cyclone is larger. But a too-strong jet has an opposite effect: the potential vorticity anomalies induced by the lower-layer

Revisiting the Atlantic South Equatorial Current

“…Figure 10b shows the Ilhéus Anticyclone being formed by the bifurcation of the 14°S band; the Royal‐Charlotte Anticyclone being formed by the BC and the 18°S SSEC band; and the Abrolhos Anticyclone being formed by the BC and the 21°S SSEC band. The latter band simply adds volume to the BC, as this boundary current reattaches itself to the continental slope south of the Vitória‐Trindade Ridge crossing (e.g., Lazaneo et al., 2020; Napolitano et al., 2021).…”

Vertical Interaction Between NBC Rings and Its Implications for South Atlantic Water Export

“…Silveira et al (2008Silveira et al ( , 2023 detail the growth of the quasi-standing cyclonic meanders off southeastern Brazil, driven by the vertical interaction of the opposing flows of the Brazil Current (BC) and the Intermediate Western Boundary Current (IWBC). Upstream on the BC path, at 20°S, Napolitano et al (2021) suggest, using a theoretical model, that the local meandering of the BC that gives rise to the Vitória Eddy (Schmid et al, 1995) is driven by vertical interaction with a topographically constrained IWBC eddy. In all of these regions, the meanders can occasionally shed coherent eddies that are then free to propagate away from the growth region.…”

“…Upstream on the BC path, at 20°S, Napolitano et al. (2021) suggest, using a theoretical model, that the local meandering of the BC that gives rise to the Vitória Eddy (Schmid et al., 1995) is driven by vertical interaction with a topographically constrained IWBC eddy. In all of these regions, the meanders can occasionally shed coherent eddies that are then free to propagate away from the growth region.…”

Topographically Generated Submesoscale Shear Instabilities Associated with Brazil Current Meanders

“…Submesoscale motions can trigger the formation of mesoscale meanders, but the opposite is also true. The interaction of mesoscale meanders with topography can ignite symmetric and inertial instabilities (Napolitano et al, 2021a), and generate Submesoscale Coherent Vortices (SCVs) (Lazaneo et al, 2022). Such processes can enhance the vertical exchange of tracers and lead to energy dissipation (Gula et al, 2016b;Lazaneo et al, 2022).…”